This invention relates to a process for preparing p-t-octyl phenol from phenol and diisobutene by catalytic alkylation.
Alkyl phenols are manufactured industrially, primarily by catalytic alkylation of phenol with olefins. Catalysis in these Friedel-Crafts reactions takes place either homogeneously (sulfuric acid, phosphoric acid, boron trifluoride, aluminum chloride, etc.) or heterogeneously (strongly acidic cation exchangers, activated aluminas, etc.). Since homogeneous catalysis causes problems regarding environmental protection and corrosion, heterogeneous catalysis with acidic ion exchangers, especially with resins of the sulfonated, divinylbenzene-crosslinked polystryrene type, is preferred. In this method, resins are employed, wherein the particle size distribution maximum is 0.3-1.3 mm. They are arranged in solid-bed reactors subjected to a flowthrough, depending on the flow velocity of the reaction mixture, from the bottom toward the top or from the top toward the bottom. The reactivity or reaction rate depends, in this connection, inter alia, on the reactants utilized, but is also dependent on the properties of the ion exchangers (degree of crosslinking, degree of sulfonation, exchange capacity, etc.). However, on the whole, despite the use of ion exchangers for catalysis on a large industrial scale, the mechanism of the reaction in dependence on the starting materials remains largely unexplored, especially in alkylations.
A step in the industrial performance of this reaction which is critical to the process is the removal of the heat of reaction. According to U.S. Pat. No. 3,422,157, this problem is solved by recycling large amounts of the reaction mixture from a first reactor back into the former via a heat exchanger, and by conducting comparatively small amounts into a second reactor. However, it is also possible, as described in French Pat. No. 2,228,749, to control the temperature by cooling coils disposed in the reactor. According to German Pat. No. 2,346,273 =U.S. Pat. No. 4,168,390=British Pat. 1,481,568), it is possible to prevent heat accumulation by affecting the activity level by the use of two series-connected reactors having different catalyst activities. This is achieved, for example, by partially replacing the hydrogen ions of the sulfo group by aluminum ions. Additionally, however, the selectivity behavior of these catalysts can be modified by exchanging H-ions with metal ions. This is frequently accompanied by a simultaneous reduction in the reaction velocity (H. Widdecke: "Ionenaustauscher als polymere Katalysatoren zur Alkylierung von Aromaten" [Ion Exchangers as Polymeric Catalysts for the Alkylation of Aromatics], Dissertation 1978, TU [Technical College] Braunschweig, page 131).
Especially remarkable is the known dependency of the heterogeneously catalyzed alkylation process on water, which latter influences the reaction velocity as well as selectivity. Thus, it has been found that when reacting phenol with propene at a reaction temperature of 75.degree. C. and with an addition of 10% by weight of water, the phenol ether content rises by a factor of 6.3 and the conversion, within the same reaction period, drops by a factor of 3.3. The ratio of mono- to dialkyl phenol, though, which is important for economic considerations, is not altered thereby (H. Widdecke, page 125). By raising the temperature to 125.degree. C. with the other conditions remaining the same, ether formation is greatly suppressed and the conversion rate is increased by a factor of 4.4.
The alkylation reactions with isobutene and its oligomers are likewise obscure. Thus, for example, surprisingly, no ogligomerization is observed in the alkylation of phenol as contrasted to alkylation of benzene with isobutene (H. Widdecke, page 118). In the first-mentioned reaction, phenol alkylation takes place exclusively. However, the reaction also differs from other alkylation reactions by the fact that transalkylation from o-tert-butyl phenol to the p-isomer is possible. In this connection, it is assumed that isomerization as well as transalkylation mechanisms are involved.
However, knowledge of these reaction mechanisms cannot even remotely explain the experimental results obtained in the alkylation of phenol with diisobutene. It has been found in this reaction that p-tert-butyl phenol is formed almost quantitatively rather than, as actually expected, primarily p-tert-octyl phenol. Again, only suppositions can be made regarding the course of this reaction. These make it appear possible that the first step is cleavage of the diisobutene molecule, or a disproportionation of the initially formed octyl phenol. The formation of butyl phenol can be suppressed by the addition of 1-2% of water, based on the amount of phenol utilized, at reaction temperatures of 85.degree.-110.degree. C., especially 100.degree.-105.degree. C. (French Pat. No. 2,228,749), the catalyst having previously been impregnated additionally with 10-15% of its weight of water. Under these reaction conditions, however, the reaction velocity, as is expected, is very low due to the inhibitory effect of the water; in addition, the proportion of undesired dialkyl phenols is relatively high. At temperatures of between 120.degree. and 130.degree. C. and without addition of water, only very small amounts of octyl phenol are obtained in the process of French Pat. No. 2,228,749, but p-tert-butyl phenol is formed in a high yield (94-96%).